Rheostat



Patented Sept. 20, 1932 UNITED STATES PATENT OFFICE RAYMOND P. MATTERN, F WAUWA'IOSA, WISCONSIN, ABSIGNOB, BY MESNE ASSIGN- MENTS, T0 ALLEN-BRADLEY COMPANY, 0F MILWAUKEE, WISCONSIN, A CORPORA- TION OF WISCONSIN BHEOSTAT Application led February 28, 1930. Serial No. 432,210.

This invention relates to certain new and useful improvements in rheostats of the type employed in connection with radio apparatus, and has as an object, the provision of novel ,5 means for providing good electrical connec-` tion between any portion of a high resistance unit and a movable Contact arm.

Another object of this invention resides 1n the provision of a resistor unit molded or east of material having a relatively high resistance value and having members of low resistance embedded therein to insure a good electrical connection between any portion of the resistor unit and a movable contact arm. Y

Another object of this invention resides in the provision of a resistor unit of the character described in which terminal members of low resistance are embedded therein in such a manner as to insure absolute and permanent stability.

A further object of this invention resides in the provision of a resistor unit of the character described which comprises integrally connected sections having different degrees of resistance.

And a still further obj ect of this invention resides in the provision of a novel resistor unit of the character described which is simple in construction and which lends itself particularly well to economical production.

With the above and other objects in view which will appear as the description proceeds,

my invention resides in the novel construction,

combination and arrangement of parts sub- 36 stantially as hereinafter described and more particularly defined by the appended claim,

'it being understood that such changes in the i precise embodiment of the hereindisclosed 4o scope of the claim.

In the accompanying drawing, I have illustrated one complete example of the physical embodiment of my invention constructed according to the best mode I have so far deinvention may be made ascome within the taken through Figure 1 on the plane of the line Q-Q;

Figure 3 is a perspective view of the resistor unit per se shown detached from the rheostat, and

Figure 4 is an enlarged perspective` sectional view of the resistor unit.

Referring now more particularly to the accompanying drawing, the numeral 5 represents the base of the rheostat which is centrally apertured, as at 6, for the reception of a bushing 7 in which an actuating shaft 8 is freely rotatably mounted. The bushing 6 is maintained in proper assembled relation with the base by a nut 9, and extends outwardly from the base to provide convenient means for mounting the rheostat from panel '10.

The outer end 11 of the shaft 8 is enlarged to form a shoulder which engages the'adjacent bushing end to prevent inward movement of the shaft, and has a control knob 12 fixed thereon. The opposite end of the shaft projects through a spring Washer 13, the inner end of a movable contact arm 14, and a pair of insulating discs 15 to which the arm is riveted or otherwise secured. A. screw 16 threaded in the shaft secures the elements in proper assembly and places the spring washer 13 in compression to maintain a' frictional drive connection between the shaft and the discs 15 carrying the movable contact arm. The resiliency of the spring washer also provides a good electrical connection between the arm and a terminal 17 formed integral with the spring washer.

The outer free end of the arm is movable over and frictionally engageable with an annular resistor unit 18 mounted in an annular recess 19 formed in the base`5. The resistor unit is molded or cast of powdered resistance material having the desired resistivity and to insure good electrical contact between it and the movable contact arm 14, a plurality of metal prongs or bars 20 are embedded in the resistor unit at the time of its formation.

For convenience in production the prongs or bars may be the teeth of a metal comb from which the back portion B (see F igure 4) is severed along the line C after the molding of the unit, as described in my c0- pending application, Serial No. 432,211 tiled Feb. 28,' 1930, and to insure intimate electrical contact between the prongs and the molded material they are preferably silver plated and coated with detiocculated graphite, prior to being embedded in the material.

' As best illustrated in Figure 3, the bars 2O are substantially entirely7 embedded in the unit and have only one end projecting slightly beyond the surface of the unit to provide a plurality of closely spaced contact surfaces extending around the entire circumference ot the unit, with which the contact arm 14 is frietionally engageable. It is also noted that the bars 20 have but a small cross sectional area and are straight and parallel with the side walls of the unit, which reduces internal stresses to a minimum and insures permanent intimacy of contact between the bars and the unit and consequently absolute stability.

At two adjacent points in the circumference of the unit. the bars 2O are replaced by terminal members 21 which have apertures 21 near their inner sides to increase the bond between them and the molded material, and the material filling the intervening space between the terminal members is cut away. Conductors 22 soldered or otherwise secured to the terminal members 21 are passed through suitable apertures 23 through the base in communication with the annular channel or recess 19 and have their end portions soldered or otherwise iixed to terminal lugs 24. The terminal lugs 24 are secured to the base by rivets 25 whose inner ends are elongated to provide stops for limiting the rotational movement of the contact arm 12.

'Ihe space between the separated ends of the unit enables it to be sprung slightly in either direction to facilitate its assembl with the base Where it is held by the inlierent resiliency of the unit. The space between the ends of the unit also provides room for it to expand and the conductors 22, besides electrically connecting the ends of the unit with the terminal lugs prevent accidental detachment of the unit from the base. By providing a terminal 24 for each end of the unit, the rheostat may be used as a potentiometer, as will be readily obvious.

If desired, the resistance unit may comprise sections of different degrees of resistivity, and the present unit is divided into four individual sections which are indicated by a diiierence in shading. To obtain this composite resistor it is only necessary to divide the forming mold into separated compartments and to till the various compartments with powder having the desired resistance values, after which the dividing means are removed and the powders compressed. During the compression of the unit, the adjacent powders merge into each other and fuse so that the step in resistance from one section to another is not abrupt.

The completed unit also may be thoroughly impregnated with wax to protect the same against moisture, and in Figure 4 a, coating of wax W is indicated.

From the foregoing description, taken in connection with the accompanying drawing, it will be readily apparent to those skilled in the art to which an invention of the character described appertains, that I provide a novel manner of assuring good electrical contact between a molded high resistance unit and a movable contact arm, and a novel construction for a resistor unit which is entirely self supporting to facilitate its assembly with and removal trom any desired apparatus, and wherein absolute stability is assured.

What I claim as my invention is:

A rheostatic resistor, comprising a split ring of molded material of high resistance value and having sufficient rigidity to be selfsupporting, a series of elongated contact members and having all portions save their outer ends permanently embedded in the ring of molded material, terminal members permanently embedded in the ring of molded material at the ends of the series of Contact members and having their outer ends exposed to provide contact surfaces and their sides exposed to facilitate the connection of conductors therewith, and material ofA high conductivity characteristics between the Contact and terminal members and the molded material to provide an intimate electrical contact between the molded material of high resistance value and the contact and terminal members.

In testimony whereof I have hereunto affixed my signature.

RAYMOND P. MATTERN. 

